Cohesin supercoils DNA during loop extrusion

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Cohesin mediates the extrusion of genomic DNA into loops, which is crucial for chromatin assembly, gene regulation, and recombination.
• The precise mechanism by which cohesin translocates DNA during loop extrusion remains poorly understood, despite its dependence on large-scale conformational changes.

Purpose of the Study:

• To elucidate the biophysical mechanism of DNA translocation by cohesin during loop extrusion.
• To investigate the role of DNA supercoiling in cohesin-mediated loop extrusion.

Main Methods:

• In vitro biochemical assays to study cohesin-DNA interactions and DNA translocation.
• Analysis of cohesin mutants defective in supercoiling.
• Cellular assays to assess DNA loop formation and genome architecture.

Main Results:

• Cohesin negatively supercoils DNA during the loop extrusion process.
• This supercoiling requires the engagement of cohesin's ATPase heads and a DNA-binding site on its hinge.
• A cohesin mutant impaired in negative supercoiling exhibited shorter DNA loops in cells, and depletion of topoisomerase I mimicked this phenotype.

Conclusions:

• Negative supercoiling is an integral component of the cohesin loop extrusion mechanism.
• The relaxation of supercoiled DNA by factors like topoisomerase I is necessary for efficient cohesin-mediated loop extrusion and the maintenance of genome architecture.